In mobile scenarios, wireless communications systems take advantage of phased array antennas that allow for an optimal steering of the radiation characteristics. The phased array is able to adapt its radiation characteristics according to the instantaneous situation. That is, the main beam of radiation can be electronically aligned towards the remote station, independent of the relative orientation between both. This leads to a high signal quality and reliable transmission without any mechanical re-orientation of the antenna.
The beam forming in a phased array relies on the phase progression along the radiating aperture. This phase progression is generated by an excitation network, which allows electronical variation of the phase of the signal to be transmitted or the signal received. A key component of phased arrays is therefore a phase shifter. The phase shifter is a two-port device that introduces a tuneable phase lag to the passing signal between the input port and the output port.
In electronically controlled phase shifters, the phase lag can be tuned via an electrical signal. Depending on the architecture, the tuning can be done continuously or in discrete steps. In continuously tuneable phase shifters, an analogue signal is applied to the phase shifter. While a continuously tuneable phase shifter provides an arbitrary phase shift, it is more sensitive to temperature variations, manufacturing tolerances and alike. Application of continuously tuneable phase shifters therefore needs means for calibration to compensate for phase errors. With a discrete tuneable phase shifter, the phase shift can only be varied within a limited set of steps, restricting the beam forming capabilities in a phased array. Yet discrete tuneable phase shifters are usually less sensitive to environmental variations or manufacturing tolerances and might therefore be easier to implement with lower calibration effort.
Implementations of electronic phase shifters have been well known for several decades. Early implementations were based on PIN diodes, which served as switching devices. FIGS. 4a to 4c show single sections of so-called switched-line phase shifters using series switches and shunt switches, respectively. FIG. 4a shows series switches, FIG. 4b shows shunt switches, and FIG. 4c shows an example implementation using switching shunt diodes D1 to D4. Examples of such phase shifters are described in R. V. Garver, “Broad-Band Diode Phase Shifters,” IEEE Transactions on Microwave Theory and Techniques, vol. 20, no. 5, pp. 314-323, May 1972; and S. K. Koul and B. Bhat, “Microwave and Millimeter Wave Phase Shifters”, Artech House, Boston, 1991, for example. With a cascade of multiple sections each representing another phase shift, the total phase shift can be altered in discrete steps. The phase shift of section n is given by ψn=k(ln−l), with k being the wavenumber and l and ln being the mechanical length of the respective line. The switched-line phase shifter therefore relies on the length variation of the path, which is passed by the signal. A switched-line phase shifter of N sections with a total phase shift of 360 degrees has a resolution of 360°/2N. Suppose a 3-bit phase shifter, i.e., N=3, the resolution amounts 45°.
Among switching between lines of different mechanical lengths, a phase shift can also be achieved by tuning of material characteristics, i.e., the electrical length of the line is altered rather than the mechanical length thereof. A line section of mechanical length l shows a phase shift of ψ=k(μr, εr)l. The wavenumber k follows from
      k    ⁡          (                        μ          r                ,                  ɛ          r                    )        =            2      ⁢                          ⁢      π      ⁢                          ⁢      f      ⁢                          ⁢              n        c              =          2      ⁢                          ⁢      π      ⁢                          ⁢      f      ⁢                                                  μ              r                        ⁢                                                  ⁢                          ɛ              r                                      c            with c the free-space velocity of light, f the frequency, n the refractive index, μr the relative permeability, and εr the relative permittivity of the substrate supporting the transmission line. A variation of μr or εr causes a variation of k and, therefore, a varying phase.
This approach was pursued in ferrite-type phase shifters, where the permeability of a ferrite material is varied by an external magnetic field applied to it, as described in S. K. Koul and B. Bhat, “Microwave and Millimeter Wave Phase Shifters”, Artech House, Boston, 1991. The drawback of ferrites is their losses, especially occurring at frequency above 1 GHz.
In recent years, non-linear dielectric materials became available and have been used for the implementation of phase shifting devices. In contrast to ferrite-type phase shifters, the permittivity is varied while μr=1. Non-linear dielectrics include so-called ferroelectrics, a solid mixture (e.g. mixtures of Barium, Strontium, and Titanate), and so-called liquid crystals (LC). Applying an electric field of proper strength to a non-linear dielectric causes a variation of the permittivity and, therefore, of the phase.
Implementations of phase shifters featuring LC mixtures rely on a continuous variation of the permittivity, such as those described in C. Weil, G. Luessem, R. Jacoby, “Tunable Inverted Tunable Inverted-Microstrip Phase Shifter Device Using Nematic Liquid Crystals,” Microwave Symposium Digest, 2002 IEEE MTT-S International (Vol. 1), 2-7 Jun. 2002, Seattle, Wash., USA, pp. 367-371; and S. Müller et al., “Tunable Passive Phase Shifter for Microwave Application using Highly Anisotropic Liquid Crystals,” Microwave Symposium Digest, 2004 IEEE MTT-S International (Vol. 2), 6-11 Jun. 2004. Variations caused by temperature variations, for example, have therefore to be monitored and considered for the biasing of the LC mixture. This holds also for ferroelectric and ferrite-based solutions. Phased arrays comprising tens or hundreds of phase shifters need much effort for calibration.